Ink jet printers have proven effective for many printing purposes. It is an ongoing goal to increase the printing speed for most printers, including ink jet printers. It is also a goal to provide excellent image quality, such as by providing an image matrix having higher resolution, or more dots per inch (DPI). With ink jet printers, increased print speed may be provided by increasing the velocity at which the print head scans over the print media; increased resolution is also often provided by printing smaller ink droplets, closer together.
However, for very high speed printing, particular at increasingly fine resolutions, ink jet printers have been found to exhibit a characteristic that limits print quality. It is believed that upon ejection of a droplet from an ordinary orifice, a tail portion of the droplet may lag the head or main droplet portion. As the elongated droplet proceeds from the print head to the media sheet, surface tension may cause the droplet to break into a main droplet and a separate satellite droplet. With moderate scan rates, this is not a concern, as the satellite strikes the media within the larger spot formed by the main droplet; even if it is not concentric with the main droplet, it is obscured by the larger main spot. However, as scan rates increase, the second droplet strikes increasingly farther from the center of the main spot. This can lead to elongated spots which exhibit an apparent lobe on one side formed by the tail droplet. At still higher scan rates, a separate spot is formed by the satellite droplet apart from the main spot. This reduces image quality by making sharp lines appear fuzzy or jagged, and by adding to locations where it is not intended. This issue is discussed in U.S. Pat. No. 5,369,428 to Maze et al. Which is incorporated herein by reference.
This problem with high speed printing is worsened with smaller ink drop volumes required for higher resolution printing. Instead of causing a proportionate reduction of main and tail drop portions, drop volume reduction has been found to primarily reduce the main droplet portion, without affecting tail droplet volume appreciably. Thus, any undesired printing artifacts generated by offset tail spots will be more noticeable and objectionable relative to the main droplets for higher resolutions than they would be for lower resolutions.
The present invention overcomes the limitations of the prior art by providing a method of ink jet printing including positioning an ink jet print head having a number of nozzles adjacent a sheet of printer media, and moving the sheet or the print head along a scan axis. An ink droplet is expelled from one of the nozzles in an ejection direction offset from perpendicular to the scan axis. The droplet may have a main portion and a tail portion having different velocities and directions, with these parameters and the rate of scanning selected so that both portions strike the same location on the media sheet.